docs: correct misleading SQ4 quantization comment in type definitions

Two type-definition comments (src/coreTypes.ts:472 +
src/types/brainy.types.ts:1123) stated:

  bits?: 8 | 4   // default: 8 (SQ8). SQ4 requires cortex native.

This was factually wrong. src/utils/vectorQuantization.ts:412 ships
distanceSQ4Js, a pure-JS SQ4 implementation that's been part of the
open-core path the whole time. The native SIMD acceleration is a drop-in
via setSQ4DistanceImplementation, NOT a hard requirement.

The misleading comment risked pushing open-core users to assume they
needed a paid native provider for SQ4 quantization when they did not.

Corrected to:

  bits?: 8 | 4   // default: 8 (SQ8). SQ4 has a pure-JS implementation;
                 // cortex's distance:sq4 SIMD provider accelerates it.

Pure documentation; no behaviour change. Caught during an upstream
open-core-boundary audit (handoff thread BRAINY-8.0-RENAME-COORDINATION,
section E.1).

Verification

- npx tsc --noEmit: clean
- npm test: 1468 / 1468 unit
- npm run build: clean

src/coreTypes.ts

@@ -469,7 +469,7 @@ export interface HNSWConfig {
   // SQ8 vector quantization (4x memory reduction, ~0.4% accuracy loss)
   quantization?: {
     enabled: boolean        // default: false — preserves current behavior exactly
-    bits?: 8 | 4            // default: 8 (SQ8). SQ4 requires cortex native.
+    bits?: 8 | 4            // default: 8 (SQ8). SQ4 has a pure-JS implementation; cortex's distance:sq4 SIMD provider accelerates it.
     rerankMultiplier?: number // default: 3 — over-retrieve 3x, rerank with float32
   }
   // Vector storage mode

src/types/brainy.types.ts

@@ -1120,7 +1120,7 @@ export interface BrainyConfig {
   hnsw?: {
     quantization?: {
       enabled?: boolean        // default: false — current behavior exactly
-      bits?: 8 | 4             // default: 8 (SQ8). SQ4 requires cortex native.
+      bits?: 8 | 4             // default: 8 (SQ8). SQ4 has a pure-JS implementation; cortex's distance:sq4 SIMD provider accelerates it.
       rerankMultiplier?: number // default: 3 — over-retrieve 3x, rerank with float32
     }
     vectorStorage?: 'memory' | 'lazy'  // default: 'memory' — 'lazy' evicts vectors after insert